Multilayer printed circuit boards are used for a variety of electrical applications and provide the advantages of weight and space conservation in electronic devices. A multilayer board is comprised of two or more circuit layers, each circuit layer separated from another by one or more layers of dielectric material. Circuit layers are formed by applying a copper layer onto a polymeric substrate. Printed circuits are then formed by techniques well known in the art. For example, patterned circuitry innerlayers are first prepared by a process in which a copper foil-clad dielectric substrate material is patterned with resist in the positive image of the desired circuitry pattern, followed by etching away of the exposed copper. Upon removal of the resist, there remains the desired copper circuitry pattern.
Once the circuit patterns are formed, a stack is formed comprising multiple circuit layers separated from each other by a dielectric layer. The one or more circuitry innerlayers of any particular type or types of circuitry pattern, as well as circuitry innerlayers which might constitute ground planes and power planes, are assembled into the multilayer circuit by interposing one or more partially-cured dielectric substrate material layers (so-called “pre-preg” layers) between the circuitry innerlayers to form a composite of alternating circuitry innerlayers and dielectric substrate material. The composite is then subjected to heat and pressure to cure the partially-cured substrate material and achieve bonding of circuitry innerlayers thereto. The thus cured composite will then have a number of through-holes drilled therethrough, which are then metallized to provide a means for conductively interconnecting all circuitry layers. In the course of the through-hole metallizing process, desired circuitry patterns also typically will be formed on the outer-facing layers of the multilayer composite.
The metallizing of the through-holes involves steps of resin desmearing of the hole surfaces, catalytic activation, electroless copper depositing, electrolytic copper depositing, and the like. Many of these process steps involve the use of media, such as acids, which are capable of dissolving the copper oxide adhesion promoter coating on the circuitry innerlayer portions exposed at or near the through hole. This localized dissolution of the copper oxide, which is evidenced by formation around the through-hole of a pink ring or halo (owing to the pink color of the underlying copper metal thereby exposed), can in turn lead to localized delamination in the multilayer circuit.
The art is well aware of this “pink ring” phenomenon, and has expended extensive efforts in developing a multilayer printed circuit fabrication process which is not susceptible to such localized delamination. It has long been known that the strength of the adhesive bond formed between the copper metal of the circuitry innerlayers and the cured pre-preg layers, or other non-conductive coatings, in contact therewith leaves something to be desired, with the result that the cured multilayer composite or the coating is susceptible to delamination during subsequent processing and/or use. In response to this problem, techniques of forming on the copper surfaces of the circuitry innerlayers (before assembling them with pre-preg layers into a multilayer composite) a layer of copper oxide, such as by chemical oxidation of the copper surfaces have been developed.
The earliest efforts in this regard (so-called “black oxide” adhesion promoters) produced somewhat minimal improvement in the bonding of the circuitry innerlayers to the dielectric substrate layers in the final multilayer circuit, as compared to that obtained without copper oxide provision. Subsequent variations and/or improvements on the black oxide technique included methods wherein a black oxide coating is produced on the copper surface, followed by post-treatment of the black oxide deposit with 15% sulfuric acid to produce a “red oxide” to serve as the adhesion promoter, such as disclosed by A. G. Osborne, “An Alternate Route To Red Oxide For Inner Layers”, PC Fab. August, 1984. Notable improvements in this art are represented in U.S. Pat. Nos. 4,409,037 and 4,844,981 to Landau, the teachings both of which are included herein by reference in their entirety.
Other approaches to this problem involve post-treatment of the copper oxide adhesion promoter coating prior to assembly of circuitry innerlayers and pre-preg layers into a multilayer composite. For example, U.S. Pat. No. 4,775,444 to Cordani discloses a process in which the copper surfaces of the circuitry innerlayers are first provided with a copper oxide coating and then contacted with an aqueous chromic acid solution before the circuitry innerlayers are incorporated into the multilayer assembly. The treatment serves to stabilize and/or protect the copper oxide coating from dissolution in the acidic media encountered in subsequent processing steps (e.g. through-hole metallization), thereby minimizing pink ring/delamination possibilities.
U.S. Pat. No. 4,642,161 to Akahoshi et al, U.S. Pat. No. 4,902,551 to Nakaso et al, and U.S. Pat. No. 4,981,560 to Kajihara et al, and a number of references cited therein, relate to processes in which the copper surfaces of the circuitry innerlayers, prior to incorporation of the circuitry innerlayers into a multilayer circuit assembly, are first treated to provide a surface coating of adhesion-promoting copper oxide. The copper oxide so formed is then reduced to metallic copper using particular reducing agents and conditions, such as amine boranes. As a consequence, the multilayer assembly employing such circuitry innerlayers will not evidence pink ring formation since there is no copper oxide present for localized dissolution, and localized exposure of underlying copper, in subsequent through-hole processing. As with other techniques described herein, processes of this type are suspect in terms of the adhesion attainable between the dielectric substrate layers and the metallic copper circuitry innerlayers.
U.S. Pat. Nos. 4,997,722 and 4,997,516 to Adler similarly involve formation of a copper oxide coating on the copper surfaces of circuitry innerlayers, followed by treatment with a specialized reducing solution to reduce the copper oxide to metallic copper. Here again, however, problems can arise in terms of the adhesion between the dielectric layers and metallic copper circuitry innerlayers.
U.S. Pat. No. 5,289,630 to Ferrier et al., the teachings of which are incorporated herein by reference in their entirety, reveals a process whereby an adhesion promoting layer of copper oxide is formed on the circuit elements followed by a controlled dissolution and removal of a substantial amount of the copper oxide in a manner which does not adversely affect the topography. Variations/improvements on this process are described for example in U.S. Pat. No. 5,869,130 to Ferrier. U.S. Pat. No. 6,020,029, also to Ferrier, offers the step of increasing adhesion by contacting the metal surface with an alkaline solution after the adhesion promoting composition is applied. Other improvements are described in U.S. Pat. Nos. 6,146,701, 6,162,503, 6,383,272, 6,419,784, 6,506,566 and 6,554,948 all to Ferrier.
As described herein, conventional black oxide coatings for bonding copper and copper alloys to resins are well known in the art.
Typical steps in a conventional black oxide process include:
(1) acid cleaner;
(2) alkaline cleaner;
(3) microetch;
(4) pre-dip;
(5) black oxide;
(6) post-dip; and
(7) hot air dry.
The microetch composition typically etches the copper substrate to a depth of approximately 40-55 microinches and comprises either sulfuric acid/peroxide or persulfate and is accomplished at a temperature of about 32° C. for about 1.5 to about 2.5 minutes. The predip composition used in this process is typically a 2-3% solution of sodium hydroxide that is applied at a temperature of between about 18-27° C. Thereafter, the black oxide coating is applied, which typically comprises a chlorite/sodium hydroxide mixture that is applied at a temperature of about 70-90° C. for a period of about 4 to about 6 minutes. The black oxide process is typically applied as a vertical (or immersion) application. Next, the black oxide coated copper is subjected to a post dip treatment, which typically comprises an amine borane, such as dimethyl amino borane, as described for example in U.S. Pat. No. 4,643,161 to Akahoski, and which is applied at a temperature of abut 35° C. for about 4-5 minutes. In the alternative, controlled dissolution of the copper can be accomplished. Finally hot air drying is performed at a temperature of about 66-93° C. for about 6-12 minutes.
In an “alternative oxide” coating process, which is typically applied in a horizontal (i.e., conveyorized) application, the steps typically include:
(1) acid cleaner;
(2) alkaline cleaner;
(3) microetch;
(4) predip;
(5) brown oxide;
(6) optional post-dip for enhanced bonding; and
(7) hot air dry.
The microetch composition typically etches the copper substrate to a depth of approximately 40-60 microinches and comprises either sulfuric acid/peroxide or persulfate and is accomplished at a temperature of about 32° C. for about 1.5 to about 2.5 minutes. The predip composition used in this process is typically a caustic or an acid/peroxide solution. Thereafter, the alternative conversion coating is applied, which typically comprises a solution of sulfuric acid, a peroxide and benzotriazole-based additives and is applied at a temperature of about 32-38° C. for a period of about 45 seconds to about 1.5 minutes. In this alternative oxide process, the post dip is optional and is used generally with high Tg materials for enhanced bonding. Finally, hot air drying is performed at a temperature of about 66 to 93° C. for 10 to 20 seconds using a turbo drying process.
The inventors of the present invention have found that it is desirable to provide further improvements to the bond between copper/copper alloy and resin in multilayer circuit board construction. It is also desirable to provide an enhanced passivate nano-oxide coating method that produces a unique bond between no-profile copper/copper alloy and resin. Finally, it is desirable to provide an enhanced coating method that provides cost savings as compared to the conventional processes and significantly reduces the waste treatment cost for the process.
The present invention describes a process for improving the adhesion of polymeric materials to a metal surface, especially copper or copper alloy surfaces. The process set forth herein is particularly useful in the production of multilayer printed circuits. The process described herein provides optimum adhesion between the metallic and polymeric surfaces (i.e., the circuitry and the intermediate insulating layer), eliminates or minimizes pink ring and operates economically, all as compared to conventional processes. Finally, the process described herein also provides an improved bond between copper and high performance resin materials.